The present invention relates to imaging systems, and more particularly to the correction of non-uniformities among detectors in multiple-detector imaging systems.
Today, optical imaging systems are in widespread use. For example, thermal imaging devices are used in diverse applications ranging from military missile-seeking systems to early fire detection and warning systems. Recent advances in imaging system technology, particularly the trend toward multiple-detector imaging systems in which an array of radiation detectors are used to capture scene images, have led to a need for more advanced detector calibration techniques. Specifically, each detector in a multiple-detector system typically must be calibrated so that the entire detector array provides a uniform output level when irradiated by a uniform input source.
As a result, numerous techniques have been developed for creating uniform reference sources which can be used to irradiate an array of detectors and to thereby determine individual detector variations across the array. Such variations are generally characterized by level and gain differences between individual detectors and are correctable once known. For example, if the output level provided by each of a plurality of detectors in response to a fixed uniform reference source is known, then an appropriate detector-specific offset value can be removed from each detector output during operation so that the entire detector array is calibrated at least at the reference level. This type of single-point calibration is depicted in FIG. 1.
In FIG. 1, output-voltage versus input-radiation-flux characteristics for three photovoltaic-type detectors are shown as three curved lines 110, 120, 130 which intersect at a single point corresponding to an input-flux reference level IF.sub.R and an output-voltage reference level V.sub.R. The curves 110, 120, 130 shown in FIG. 1 result, for example, when the three photovoltaic detectors are calibrated by: irradiating the detectors with a uniform reference flux of value IF.sub.R ; measuring the output level for each detector in response to the uniform input flux IF.sub.R ; computing an offset value for each detector as the difference between the measured output value and the reference output value V.sub.R ; and thereafter removing the computed offset values from the respective detector outputs. The reference output V.sub.R to which the detectors are calibrated can be computed, for example, as the mean, median or mode value output by the detectors in response to the uniform reference flux IF.sub.R. As shown in FIG. 1, single-point calibration forces the detector outputs to be uniform at V.sub.R for the single reference input flux IF.sub.R, but does not necessarily provide uniformity of output for other input flux levels.
Improved output uniformity can be achieved using two-point calibration as shown in FIG. 2, wherein output-voltage versus input-flux characteristics for three photovoltaic detectors are shown as three curved lines 210, 220, 230 intersecting at two points. The first point of intersection corresponds to a first input-flux reference level IF.sub.R1 and a first output-voltage reference level V.sub.R1, and the second point of intersection corresponds to a second input-flux reference level IF.sub.R2 and a second output-voltage reference level V.sub.R2. The curves 210, 220, 230 shown in FIG. 2 result, for example, when the three photovoitaic detectors are calibrated by: irradiating the detectors with a first uniform reference flux of value IF.sub.R1 and measuring the resulting output level for each detector; subsequently irradiating the detectors with a second uniform reference flux of value IF.sub.R2 and measuring the resulting output level for each detector; computing the first and second output reference values V.sub.R1, V.sub.R2 for example as the mean, median or mode values output by the detectors in response to the first and second uniform reference fluxes I.sub.R1, I.sub.R2, respectively; and using the reference values V.sub.R1, V.sub.R2 and the measured detector outputs to compute a scale factor and an offset value for each detector which will force the detector outputs to the first and second reference values V.sub.R1, V.sub.R2 for input fluxes corresponding to the first and second reference fluxes I.sub.R1, I.sub.R2, respectively.
Since typical detector output characteristics are nearly linear over a given operating range, two-point calibration can be used to obtain very closely matched curves throughout that operating range. In other words, if multiple detectors are precisely calibrated at two reference levels within a region in which the detectors operate in an approximately linear fashion, then the detectors will also be very nearly calibrated throughout the linear region. In FIG. 2, the three operating curves 210, 220, 230 are precisely matched at the first and second reference levels I.sub.R1, I.sub.R2 and are very nearly matched in a linear operating region lying between the two reference levels. For best accuracy, the reference levels are positioned near the extremes of the linear operating region. Note that, although it is not generally efficient to calibrate a detector array using more than two reference points, multiple reference levels can be used to make extremely precise piece-wise linear corrections.
Thus, if an array of detectors can be irradiated with one or more uniform reference sources, then the array can be calibrated in a relatively straightforward fashion. However, providing such uniform reference sources has proven difficult. For example, one cannot simply position a reference source in the ordinary field of view of an imaging device, as doing so results in unacceptably high uniformity requirements for the reference source itself. In other words, if the reference source is positioned in a viewed scene such that it is focused on the detector array, then the reference source must be truly uniform in order to provide uniform radiation at the detector array. It is therefore typically preferred that the reference source be positioned at a non-imaging location in the optical path of the imaging device so that the reference is not focused on the detector array. Doing so creates a natural defocusing and averaging effect which results in a uniform photon flux across the detector array without requiring that the reference source itself be precisely uniform. However, selectively positioning a reference source within a tightly confined and difficult to access optical path also presents challenges.
Furthermore, controlling the magnitude of a reference photon flux has proven to be far from trivial. This results from the fact that it is typically preferable that reference fluxes be continually adjusted in dependence upon the level of energy emanating from an ever changing viewed scene. For example, empirical studies have demonstrated that best corrections can be achieved if the two reference sources in a two-point calibration are dynamically adjusted to bound the instantaneous average scene photon flux. Alternatively, one reference source can be positioned at the instantaneous average scene flux while the other reference source is positioned somewhere above or below the instantaneous average flux. In either event, both reference points should be continually positioned within the instantaneous dynamic flux range of the detectors. The dynamic nature of the calibration problem is depicted in FIG. 3, wherein the three input-output curves 210, 220, 230 of FIG. 2 are again calibrated at the two input flux reference levels IF.sub.R1, IF.sub.R2 so that the curves are very closely matched in the region between the reference levels and not necessarily well matched in the regions 320 lying outside the reference levels. In FIG. 3, however, the two input flux levels IF.sub.R1, IF.sub.R2 are positioned near opposite ends of an instantaneous detector dynamic flux range 310 which, as indicated by a two-headed arrow 315, changes with time in dependence upon the viewed scene energy. As a result, the two input flux reference levels IF.sub.R1, IF.sub.R2 should be continually adjusted in time as well.
While many techniques have been developed for providing reference sources in the above described context, each known method includes significant disadvantages. For example, certain conventional systems employ controlled heating and cooling to force the photon flux at a detector array to be uniform and near the average scene flux. However, the control circuitry and power required to provide such heating and cooling is prohibitively costly and makes insertion of a reference source into the detector optical path extremely complex and expensive. Other conventional systems employ fixed thermal reference sources which are less costly and more easily inserted into the detector optical path. However, because such fixed reference sources must span the entire range of possible scene flux levels, these systems utilize a relatively large detector dynamic flux input range. As a result, the thermal sensitivity of the detector arrays employed in such systems is significantly degraded.
Thus, known techniques are unsatisfactory in many respects and there is a need for improved methods and apparatus for providing uniform radiation reference sources in multiple-detector imaging systems.